In many light oil (32.degree.-40.degree. API) reservoirs and in some medium oil (20.degree.-32.degree. API) reservoirs, most of the original oil in place (OIP) is recovered in three stages. In the initial stage, usually termed primary production, oil either flows from the wells due to the intrinsic reservoir pressure or is pumped from the reservoir. Ordinarily, only a limited quantity of the original OIP can be produced by this method, very roughly 20% of the original OIP. Waterflooding, a secondary recovery technique, is the next stage in this sequence and yields additional oil, very roughly an additional 30%. At about this percentage, the cost of continuing the waterflood usually becomes uneconomical relative to the value of the oil produced. Hence, as much as 50% of the original (OIP) can remain even after a reservoir has been extensively waterflooded. Thus, the need for tertiary recovery which is the final stage in the sequence. This stage may utilize one of several enhanced oil recovery methods; e.g., polymer flooding, CO.sub.2 flooding and fireflooding (in-situ combustion).
Fireflooding by in-situ combustion in air has received increased attention as a tertiary recovery process since it offers many distinct technical advantages over attempted competing tertiary oil recovery processes. In this technique, ambient air is pumped into the reservoir which combusts the heavier (least desirable) portions of the crude oil. The heat and gases from combustion pressurize the reservoir and decrease the viscosity of the crude oil by heating and cracking. Additional drive is imparted by the condensing steam from combustion and the hydrocarbon gases that evolve from the cracking reactions.
Fireflooding can be applied to reservoirs having oil gravities ranging from about 10.degree.-40.degree.API. Oils near 40.degree.API often have such high primary and secondary recoveries that the remaining concentration of oil, i.e., the OIP is frequently judged to be so low that air fireflooding would not be technically feasible. The usual range for fireflooding falls between about 12.degree.-32.degree.API. In particular, it has been found that light crude oils (27.degree.API and higher) often deposit insufficient coke to support in-situ combustion in air. Even when sustained, the quality of combustion is often marginal, resulting in the inefficient utilization of oxygen. The presence of high concentrations of water in the oil formation, due to the waterflooding process, aggravates the problem of achieving stable combustion because peak combustion temperature is reduced. Therefore, as proven crude oil reserves have been increasingly depleted, oil field operators have turned to secondary and tertiary recovery techniques to recover additional oil from existing reservoirs. This has often led to an overemphasis of waterflooding at the expense of a subsequent tertiary recovery process such as fireflooding, largely due to the perceived lower cost of waterflooding. As noted above, if the tertiary recovery process is fireflooding, the OIP remaining after extensive waterflooding may be too low for the fireflood to be a technical or economic success. There have been few attempts to optimize the waterflooding/fireflooding sequence even in a semi-quantitative manner. Furthermore, the advantages of using O.sub.2 enriched air to fireflood a previously waterflooded field have not previously been recognized.
In present conventional practice, waterflooding is usually continued until the cost per barrel of oil recovered becomes unacceptably high. At this point the field may be abandoned or tertiary oil recovery by in-situ combustion in air may be attempted. If the previous waterflooding has been continued to an extent leaving a low value of oil in place (OIP) remaining (below 0.1), fireflooding by combustion in ambient air has not been found technically or economically practical. Thus, the oil remaining after the waterflood has been generally considered as effectively non-recoverable.